Electrical filters are well known in the electronic art. In general, such filters are typically either bandpass, lowpass, highpass, and band reject or notch filters. Each of these filter types, in any particular application, will be required to have certain operational or functional characteristics such as cutoff frequencies, bandwidths, Q-factor etc., that will each be affected by many factors including, for example, whether or not the components that comprise the filter are active or passive components, the topology or physical arrangement of components on a circuit board, etc.
At high frequencies, i.e., over 200 MHz, so-called transmission line elements are frequently used to perform signal filtering. So-called stripline or microstrip designs, in general, have improved performance characteristics compared to filters using so-called lumped elements (discrete resistors, capacitors, inductors, as well as active circuitry) because these microstrip and stripline filters are constructed of transmission lines the electrical lengths of which determine whether or not a particular filter will be a bandpass, a band stop, a lowpass or a highpass, etc., and which thereby avoid problems such as parasitic, and stray, inductance, capacitance, and resistance.
A microstrip filter is generally considered to be a layer of conductive material, frequently considered a ground plane or reference plane, on one side of a dielectric substrate, where such as a dielectric substrate has the opposite side of the circuit board coated with conductive material, the physical lengths of which closely correspond to electrical lengths of transmission line at radio frequencies of interest. In a sense, a microstrip filter is merely a length of conductive material on a dielectric substrate that typically forms either a quarter wavelength or half wavelength transmission line for a particular signal. This length of conductive material is deposited on to one surface of a dielectric substrate (usually a circuit board), the opposite surface of which has coupled to it a substantially continuous plane of conductive material, which is usually coupled to ground.
A stripline filter, is generally considered to be a layer of conductive material sandwiched between two dielectric layers, which dielectric layers are themselves sandwiched between two conductive layers. In a stripline filter, layers of dielectric material have running through them, the signal conductor or signal trace, where upon both the outer surfaces of the dielectric layers are coated with conductive material. A stripline filter that has the signal layer sandwiched between two dielectric layers that are sandwiched between conductive layers provides an inherent improvement over a microstrip filter in that signals on the conductive layer in a stripline filter are electrically shielded by both the upper and lower conductive layers on the dielectric material. (Although a microstrip filter does not provide the signal shielding that a stripline provides, a microstrip filter is considerably easier to manufacture and hence has a lower inherent cost.)
In both the microstrip and stripline filters, the Q factor or quality factor of the filter is directly proportional to the geometry and construction of the signal layers. In both microstrip and stripline filters, a higher Q factor can be most easily realized by increasing the thickness of the signal layer, in part because of resistive losses, (which are reduced with thicker layers of conductive material) that which are proportional to the amount of material available upon which signal can be conducted.
In fabricating both microstrip and stripline filters, there is an upper limit above which Q factor might not be increased by increasing the thickness of the conductive signal layer. Since conductive signal layers in both microstrip and stripline filters are typically screen printed using well known techniques in the art, the thickness of the layer of material that can be screen printed is limited by the viscosity of the paste and by the desired accuracy and registration of the patterning of the signal layers that may be realized in a screen printing process. As the desired thickness of a film that is to be screen printed increases, (thereby permitting increases in conductive layer thickness) the paste material that is screen printed on to a substrate may begin slump as the viscus material sags both before and during its curing process. The slumping of this viscus paste material that comprises the conductive layer after its curing, decreases the accuracy and registration of a desired pattern which in turn deteriorates the desired Q factor of the filter as well as other operating characteristics of the filter.
A high frequency microstrip or stripline filter that improves the Q factor available over that which is attainable using prior art screen printing techniques would therefore be an improvement over the prior art.